In bremsstrahlung-isochromatic-spectroscopy (BIS), which is also referred to as inverse photoemission spectroscopy, one can determine the unoccupied band structure of solid bodies and their surfaces.
The surface is irradiated with electrons and the emitted ultraviolet photons are detected. Energy selection is effected by forming the ultraviolet transparent window as a low-pass element and selecting the detector gas as a high-pass element. The combination of the two thus constitutes a band filter.
Conventional detectors comprise the following combinations of window material and gas fillings. With them, photons of the indicated energy can be detected.
______________________________________ Window Gas Photon Energy (eV) ______________________________________ CaF.sub.2 I.sub.2 + Inert 9.7 .+-. 0.35 SrF.sub.2 I.sub.2 + Inert 9.28 - 9.43 CaF.sub.2 CS.sub.2 10.08 CaF.sub.2 (CH.sub.3).sub.2 CO 9.9 ______________________________________
As will be noted from the foregoing, the gas filling of the conventional detectors is often constituted from a vapor of a liquid or solid (iodine). This is disadvantageous because the vapor pressures of such material is not only temperature dependent, because of the fact that the substances derive from materials which may be liquid or solid at room temperature, a stabilization of the temperature is required to maintain constant partial pressure of the counter gases and thus a reproducible detection sensitivity.
The detectors which utilize iodine as a counter gas have the further drawback that iodine is very corrosive. The useful life of pumps, valves and other components of the vacuum apparatus is thereby reduced. With those detectors or counters in which CS.sub.2 is used, special precautions have to be taken because the carbon disulfide is highly poisonous. The detector may have to be used in a hood or the like.